The common backlight source for LCD is a cold cathode fluorescent lamp (CCFL). The CCFL is a discharge lamp composed of low-pressure mercury. Since the CCFL does not have the filaments that emit light with heat, it has longer lifetime and consumes less power than typical hot-cathode type lamps. As the size of the LCD flat panel increases, multiple CCFL lamps are required in order to provide sufficient backlight. Accordingly, it is important that the driving current is maintained within a reasonable tolerance range, 6 mArms +/−5% (or +/−0.3 mArms).
U.S. Pat. No. 6,879,114 to Jales et al., titled “Fluorescent lamp driver circuit”, discloses a driver circuit for controlling a plurality of fluorescent lamps and a plurality of transformers. However, a plurality of simultaneous switch-on and/or switch-off signals consume a great amount of power and create ripples in the power source. Therefore, the whole system may be unstable due to these “power noises”. The disclosure of this invention is herein incorporated by reference.
A solution to the above problem is to use a control system to coordinate the operations of switch-on and/or switch-off signals. U.S. Pat. No. 6,778,415 to Lin, titled “Controller electrical power circuit supplying energy to a display device”, discloses a controller which controls at least two power inverters comprising a pulse generator and a selector. The pulse generator generates a pulse signal to trigger the first power inverter. Then, another pulse signal is passed to the next power inverter by the first power inverter. The selector generates a reference voltage for those power inverters. The controller is used to provide phase shifts to the power inverters. Through the phase shift signals that are sequentially transported by each power inverter, the frequency of the periodic phase shift signals is reduced by the factor of the number of the power inverters. However, the selector circuit utilizing a superposition method based on the values of an input voltage, a reference voltage and three resistors causes higher power consumption and interferences between the regulator, the input circuit, and the output circuit. The disclosure of this invention is also incorporated herein by reference.
U.S. Pat. No. 6,707,264 to Lin et al., titled “Sequential burst mode activation circuit”, discloses a sequential burst mode activation circuit comprising a pulse modulator, a frequency selector, and a phase delay array. This circuit is mainly used for the dimming function of a plurality of fluorescent lamps. A plurality of phased pulse width modulation (PWM) signals is used to regulate the power of respective loads such that at least two loads do not turn on concurrently. However, the phase array that comprises a selection of circuitries, phase delay generators and phased burst signal generators, complicates the whole driving system of the fluorescent lamps. Thus, there is still room for improvement. The entire disclosure of this invention is also incorporated herein by reference.
FIG. 1 describes the pulse width modulation (PWM) signals for driving the inverter of the fluorescent lamps in the prior art. For example, there are two switches in an inverter of fluorescent lamps, i.e. a push-pull inverter. The push-pull inverter, which is also called push-pull converter, switches on one of the two transistors Q1 and Q2 alternately to cause a transformer core to change voltage polarity. Another type of inverter, called half-bridge inverter, uses two transistors to implement the power circuit design. In FIG. 1, a positive driving signal 11 and a negative driving signal 12 are in the form of periodic waveforms. They drive the transistor Q1 and Q2 respectively. In a push-pull design, if the transistor Q1 is a PMOS, then the transistor Q2 may be an NMOS. On the contrary, if the transistor Q1 is an NMOS, then the transistor may be a PMOS. This is the same if the bipolar junction transistors (BJT) are used. Thus, the driving signal 11 is necessary to switch on the NMOS while the driving signal 12 is to switch off the PMOS. Furthermore, the driving signal 11 is necessary to switch off the NMOS while the driving signal 12 is to switch on the PMOS. For a multiple lamps system, the signal 13 and signal 15 perform similar functions as the driving signal 11. The signal 14 and signal 16 perform the same function as the driving signal 12. In another aspect, only one PWM signal, i.e. signal 11, is used to drive a power circuit of fluorescent lamps when a class E amplifier is employed in the circuit design. Thus, the signals 11, 13 and 15 are sufficient to drive a plurality of fluorescent lamps.
Alternating current created by the resonance of a transformer is usually used to drive a fluorescent lamp. In a power inverter design, one or more transistors are employed to correct the resonant frequency of the transformer by charging the magnetic core from the power supply or discharging the magnetic core to the ground. The PWM signals mentioned above are used to control the charge and/or discharge operations of the power inverter. As a result of the charge and discharge operations, the current reaches a maximum value when the power source provides current to charge the core of the transformer, and reaches a minimum value when the transistor discharges the core where no current is consumed. The waveforms 18, 19 and 110 represent the current consumption of each fluorescent lamp in a multiple lamps system. The waveform 17 represents the total current consumption of the waveforms 18, 19 and 110. As the number of lamps used in a lighting system increases, the difference between the maximum and the minimum value of the total current consumption also increases. This phenomenon causes the system to be unstable especially in a mobile system where the power source is from a battery.